All were white, in their 20s and working as prostitutes. All went missing after meeting clients they found on Craigslist or other Internet sites. The way they were killed was "substantially similar," said Suffolk District Attorney Thomas Spota, but police did not describe the means.

"I think it [the case] fits within the known definition of what a serial killer would be," said Spota, who joined Police Commissioner Richard Dormer at a news conference at police headquarters.

Neither would describe what other progress has been made in finding the women's killer, but they appealed for outside help - particularly from people involved in the prostitution trade.

Police working with cadaver-sniffing dogs found the skeletons on Dec. 11 and 13, all wrapped in burlap and left unburied off an isolated, barrier-island road within a quarter-mile of each other. All were positively identified through a DNA match with family members.

One of the newly identified victims, Maureen Brainard-Barnes, 25, of Norwich, Conn., was last seen alive in Manhattan on July 9, 2007, police said. Police believe the killer left her body in Gilgo Beach shortly after that.

Another victim, Melissa Barthelemy, 24, went missing from her Bronx apartment nearly two years to the day later, on July 12, 2009.

City investigators listed Barthelemy as an involuntary missing person after her every-other-day call to her family abruptly ended, raising alarm, the law enforcement source said.

Shortly afterward, her sister received a call from a man using Barthelemy's cell phone, referring to her as a "whore," according to the source.

Another victim and the only one of the four who lived on Long Island, Amber Lynn Costello, 27, was last seen alive Sept. 2, 2010, in North Babylon, where she lived shortly before her death.

Officials last week identified one of the bodies as MeganWaterman, 22, of Scarborough, Maine. Waterman disappeared June 6 after traveling to Long Island to meet clients.

Costello was never reported missing, police confirmed yesterday.

David Van Norman, an expert on identifying missing persons and the deputy coroner investigator with the San Bernardino County Sheriff-Coroner's Department in California, said missing women working as prostitutes are much more likely to go unreported than others. If they are found dead, their remains are also likely to go unidentified for longer periods of time.

Spota said sex workers should be on "very high alert" as the search for a suspect or suspects continues. Craigslist officials have cooperated in the investigation, he said.

Advocates for sex workers in New York Monday said they would put out alerts about the developments in the Gilgo Beach case.

"Sex workers are vulnerable to violence every day," said Sienna Baskin, co-director of the Sex Workers Project of the nonprofit Urban Justice Center.

The first body found on Gilgo, now known to be Barthelemy, was discovered during a search for the possible remains of another missing women who worked as a prostitute, Shannan Gilbert, 24, of Jersey City.

The discovery triggered a massive search of the barrier island that turned up three more skeletons. Gilbert proved not to be among them; she remains missing and her fate is unknown.